Materials:

1. MATERIALS & METHODS ACKNOWLEDGEMENTS REFERENCE Growth stages of 12 rice cultivars (Oryza sativa L.) expressed in DD50 thermal heat units Nora T. Watson, Paul A. Counce and Terry J. Siebenmorgen INTRODUCTION • Methods (cont.): • Weather data from 2002 was obtained from the USDA weather station at Stuttgart, AR, which included average daily high and low temperatures in oF, the two data points necessary to calculate DD50 with the following equation: DD50 = (Daily Max. Temp. (oF) + Daily Min. Temp. (oF)) - 50 2 Where • Daily Max. Temp. = 94oF if maximum temperature is >94oF • Daily Min. Temp. = 70oF if minimum temperature is > 70oF From 1996 to 1999, a new developmental staging system was developed for rice (Counce et al, 2000). It would be useful to determine the amount of thermal time necessary to reach each developmental stage so that producers can use the system to determine when to apply pesticides and fertilizers Objectives • To determine the amount of DD50 units required to reach each growth stage of the new Counce et al (2000) rice developmental staging system for 12 rice cultivars • Materials: • Twelve rice cultivars (Starbonnet, Cypress, Wells, Bluebonnet, Lemont, Mars, Carolina Gold, Magnolia, Drew, Guichao, C-GL-13, and Lagrue) were planted in randomized complete block design experiment with 4 replications.. • Methods: • The growth stage of each plant, using the system of Counce et al (2000), was determined on nine plants per plot. RESULTS 2 1 Fig. 1. Graphic representation of the thermal degree days (DD50) needed to reach the (1) vegetative and (2) reproductive growth stages (Counce et al, 2000) of 12 rice cultivars. Table. 1. Average degree days (DD50) required to reach each growth stage for 12 cultivars. SIGNIFICANCE OF FINDINGS With this staging information, a producer may use the new rice staging system for improving nitrogen and pesticide application timing, and decisions related to water management.  The DD50 program and the growth staging system are interchangeable.  R2 and VF occur at the same time.  R3 is equivalent to heading, R0 is equivalent to panicle initiation and R1 very close to IE=1/2 inch.  Using the timing intervals, the flexibility of the DD50 program can be greatly increased.  If you miss an emergence date for a field, the date for starting DD50 accumulations could be reset at V1, V2, V3 instead.  Therefore, instead of just guessing at an emergence date, a grower can find the actual growth stage and time from that date and stage of development. This research was supported by a grant from the Arkansas Rice Research and Promotion Board. We thank Rachel Hillman for her excellent note-taking during this study. Counce, P. A., T. C. Keisling and A. J. Mitchell. 2000. A uniform, adaptive and objective system for expressing rice development. Crop Science 40:436-443.